Unit 2.5 Lecture 2 Flashcards
What are lac and mal operons used for?
to metabolize two nutrients (lactose or maltose)
When do bacterial cells need to synthesize proteins in the mal and lac operon that break down sugar?
if these sugars, lactose and maltose, are present
What is lactose?
disaccharide of galactose and glucose
What is maltose?
disaccharide of 2 glucose molecules
A bacteria has a mutation removing the lacZ start codon. What will happen if this bacteria is transferred from media with only glucose to media with only lactose?
lac operon will be turned on and bacteria will stop growing
What happens when there is low lactose?
no need for LacY or LacZ, LacI represses lac operon
What happens when there is high lactose?
LacY or LacZ is important for lactose metabolism, LacI binds lactose, released from operator, operon transcribed
Describe positive regulation. What happens if MalT is removed?
MalT protein activates transcription by helping RNA POL to bind
if MalT is removed, RNA POL cannot bind and no transcription occurs
Describe negative regulation. What happens if LacI is removed?
LacI protein represses transcription by blocking the binding of RNA POL
if LacI is removed, RNA POL can bind and transcription can occur
What is an operon?
cluster of genes encoded on a single mRNA but encoding several proteins
What is polycistronic mRNA?
encodes more than one protein
How is a prokaryotic promoter sequence different from an operator sequence?
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What does LacI protein do?
binds to operator but sometimes repressor releases from the operator even when lactose is absent
What happens during the brief time when LacI is not bound to the DNA?
RNA POL can transcribe the operon and the level of expression is low (basal level)
What is maltose?
a glucose disaccharide broken down by MalP and MalQ enzymes that produce energy
When does the cell need to express the MalP and MalQ enzymes?
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What is the mal operon regulated by?
an activator called MalT that is constitutively expressed
When is the mal operon off?
when maltose is low
Where is the mal operator?
upstream of promoter
RNA POL can’t bind the promoter easily on its own
-
When is mal operon on?
when maltose is present
Describe positive regulation of the mal operon.
- MalT binds maltose
- MalT changes shape
- MalT binds the operator
- RNA POL is recruited
- transcription and translation of MalP and MalQ occurs
- MalP and Mal Q break down maltose
How does MalPQ operon recruit RNA POL?
the promoter is weak, so MalT is required to recruit RNA POL
A mutation destroys the mal operator. Compared to a wild-type cell, what will the mutant phenotype be in the presence of maltose?
mutant will have less MalP production, compared to wild type
mutant knocks out MalPQ operon’s operator
Sketch the curve of the ratio of active MalT to inactive MalT over time inside a bacterial cell before and after maltose is added to the cell. Assume that the amount of maltose added is limited and it is all consumed within the time shown. Explain what is happening to the MalT protein over time and how transcription of the mal operon changes.
- active form of MalT would increase very quickly once maltose is added and starts binding to the inactive form of MalT, converting to active form that turns on MalPQ operon
- once almost all MalT is activated, the graph would level off (maximum mal transcription)
- eventually the maltose concentration will decline due to the activity of MalP and MalQ, and the ratio of active to inactive MalT would decline until it reached the starting levels (decreasing mal transcription)
What is an anabolic process?
pathways that construct molecules from smaller units
ie. arg operon: makes arginine amino acids
What is a catabolic process?
breaks down molecules
ie. lac operon: breaks down lactose into glucose and galactose
ie. mal operon: breaks down maltose into two glucose molecules
What happens if ArgR is an activator protein?
- it will help recruit RNA POL when arginine is absent
- when arginine binds to the ArgR protein, changing its shape, ArgR cannot bind the operator
What happens if ArgR is an repressor protein?
- it will block RNA POL when arginine is present
- arginine binds to the ArgR protein, changing its shape, and allowing ArgR to bind the operator
